1. Field of the Invention
The present invention relates to an excimer laser processing method and apparatus. More particularly, it relates to an excimer laser processing method and apparatus, wherein the end of the processing is suitably judged and the presence of defects is judged during the processing, in excimer laser ablation processing for forming via holes in a resin film by irradiation of an excimer laser.
2. Description of the Related Art
In recent years, attention has been drawn to ablation processing using an excimer laser as a new processing technique.
An excimer laser is a gas laser using excitation of a rare gas and a halogen and emitting a high intensity ultraviolet beam. Ordinary substances exhibit a strong absorption of ultraviolet beams, and therefore, when irradiated by a strong light pulse (e.g., about 100 MW/cm.sup.2) such as of excimer laser light, chemical bonds are destroyed in an instant and the surface layer evaporates (ablation). This phenomenon is used in excimer laser processing. There are numerous combinations of rare gases and halogens, but those used most often for processing materials are KrF (wavelength of 248 nm), XeCl (wavelength of 306 nm), and ArF (wavelength of 193 nm).
In laser processing, conventionally use has been made of the heat energy of infrared beams such as YAG and CO.sub.2 lasers. However, such processing results in large surrounding heat damage. The beam is focused for the processing, but it is difficult to make the diameter of the beam spot sufficiently small since infrared light has a long wavelength. Also, since spot processing is involved, the throughput cannot be improved.
As opposed to this, an excimer laser mainly uses the process of breaking chemical bonds by light; therefore, the processed cross-section is clean in finish. Also, it is possible to process a relatively wide area (e.g., up to about 10 mm.sup.2) at one time. Consequently, a fine pattern can be easily formed by exposure through a mask. (See for example T. A. Znotins et al.; Laser Focus, 54, May 1987, Ishizaka: Applied Mechanical Optics, September 1990 issue, etc.)
An excimer laser can be used for processing or marking metals or ceramics, but is most impressive when it comes to processing polymers. Studies are under way on applications to polyimides and other difficult-to-process polymers.
Among the fields where attention is being drawn to the usefulness of ablation processing by excimer lasers is the formation of holes (i.e., via hole processing) for connecting wiring between layers in layer insulation films of multilayer wiring substrates. Circuit substrates are being constructed of multilayers to enable higher density packaging of electronic components, and circuit patterns are being made finer as well. Therefore, even in circuit substrates, attention is being drawn to the thin film method of laying wiring one layer at a time in the same way as multilayer wiring is done in semiconductor devices. The thin film method has been long known to be suited for high density packaging, but there have been few cases of actual application due to the difficulty in formation of via holes for connection between layers. Due to the requirements for heat resistance at the time of soldering, use is made mainly of polyimide, which has a superior heat resistance, as the thin film insulating material. However, in the thin film method, the thickness of the insulating film on the circuit substrate is several tens of microns, which is an order of thickness greater than that of the semiconductor device. Accordingly, it was difficult to form fine holes in a difficult-to-process material like a polyimide by a reliable, convenient process.
For example, in a process for etching using pattern exposure of a heat curing material, problems such as difficulty of light penetrating to under the film; holes of only up to 3 times the size of the thickness of the film being able to be formed at most in wet etching by a solvent; a reverse tapering of the hole in sectional shape, making miniaturization impossible, exist.
In the dry etching process where a resist film is formed on the resin film, finer holes can be formed; however the process is complicated. Further, it is difficult to find a resist suited to a polyimide (one with large selectivity in etching).
As opposed to this, ablation processing by an excimer laser enables formation of holes with an extremely good shape faithful to the mask by a process of just exposure. Consequently, this method has been coming under focus as a good technique for making via holes in difficult-to-process films.
One of the problems in the case of forming via holes is how to monitor and manage the progress in processing. In excimer laser processing, the resin film is etched to a thickness corresponding to the intensity of the light with each shot. The processing proceeds until the underlying electrodes are exposed. Accordingly, when the number of irradiation shots is insufficient, the intensity of the light drops for some reason or another, or the film to be processed has locally thicker areas, the electrodes will not be exposed and defective connection between layers will result. On the other hand, when the number of shots is excessive, time is wasted in processing, the under lying electrodes will be damaged, and the heating of the electrode metal will cause heat deterioration of the resin film.
Therefore, monitoring the state of progress of processing is necessary to enable processing to be ended at the required sufficient number of shots. In the past, the number of irradiation shots had been set by determining certain conditions in advance; however, the number of necessary irradiation shots varies due to fluctuations in the output of the laser, changes in the distribution of light intensity in the beam, deterioration of the optical components, and variations in film thickness. Thus, it was necessary to frequently reset the conditions.
On the other hand, an important process in the processing of via holes is inspection for defects. An inspection process is essential regardless of the method adopted for making the via holes. In particular, in a multilayer thin film circuit, when defects in the lower layer are overlooked, in many cases all the processes applied on top of it become wasted. Therefore, stringent checks not allowing any mistakes become necessary. In many cases, this requires more steps than the process for forming the via holes itself and necessitates expensive facilities. There are various types of processing defects, as illustrated in FIGS. 1(a)-1(d), which may occur in, for example, a multilayer thin film circuit having a substrate 100 and a conductor 101, which can be roughly classified into the following:
(a) No via hole is made at the location where such a hole should inherently be made (see FIG. 1(a)).
(b) The via hole does not reach to the underlying electrode (conductor) (see FIG. 1(b)).
(c) A hole is made at location where no hole should inherently be made (see FIG. 1(c)).
(d) The via hole and underlying electrode (conductor) are deviated in position with each other (see FIG. 1(d)) in, for example, a multilayer thin film circuit, having a layer to be processed 102 and a layer after processing 103.
Various optical techniques for inspecting for defects involving the detection of light reflected from the surface of the insulating material, fluorescence from the insulating material, etc. and extraction of contours using the same, have been developed and put into practical use. However, with contour extraction of reflected surface light, it is difficult to detect the defects of FIG. 1(b) and (d), although the defects of FIG. 1(a) and (c) can be detected. When the insulating material is transparent and the circuit consists of only a single layer, it is possible to inspect for deviations in position, as shown in FIG. 1(d), by using light reflected from the supporting substrate 100, such as Si, and light reflected from the insulating film, for pattern recognition and comparison of the two reflected lights against each other. However, when the circuit consists of multiple layers, the wiring of the layer further below the wiring layer where the via holes are formed becomes an obstacle, making inspection difficult.